We have compared the function of the human α-, β- and δ-globin genes using various plasmid expression vectors derived from pBR322. Amplification of recombinants occurred after their introduction, by calcium-phosphate-mediated DNA transfer, into monkey kidney cells that constitutively produce T antigen (COS cells). The human α-globin gene promoter functioned independently, but the β-globin gene promoter was nearly totally dependent on the enhancing activity of the 72 bp direct repeats from the SV40 genome. Furthermore, when the human α- and β-globin genes were linked in the same vector, the a promoter was active but the β promoter was not. Function of the δ-globin gene promoter also depended on the enhancer element. In vectors containing the 72 bp repeats and the β- or δ-globin gene, the activity of the β-globin gene was approximately 50 times greater than that of the β-globin gene, approximating the ratio of β and δ mRNA observed in normal human bone marrow cells.